JP2007016259A - System for collecting gold while recycling iodine ion in gold-removing liquid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce an environmental load due to a cyanide ion contained in a conventional gold-removing liquid used when collecting gold from discarded materials containing a substrate of an electronic circuit. <P>SOLUTION: This system for refining and collecting gold comprises the steps of: removing gold by using a gold-removing liquid containing iodine and iodine ions; reducing and precipitating the resultant gold compound into gold, by using zinc; extracting gold with aqua regia; and back-extracting gold into gold oxide by using sodium hydroxide; and reducing the gold oxide. The system also includes the steps of: making an anion-exchange resin adsorb the iodine ions formed in a reduction step by zinc to separate them from an aqueous solution containing them; returning the iodine ions to the gold-removing step; and reusing them in the step. Thereby, the system can reduce the environmental load because of using the iodine ion in place of the cyanide ion, and can reduce a cost of a raw material for the iodine and a waste water treatment charge because of recycling the used solution containing iodine ions. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for recovering valuable metals in industrial waste, and more particularly to a method for peeling and recovering gold or a gold alloy from waste electrical circuit boards.

  Cyanide ions have been widely used in the past as eluents for precious metals from ores, plating complexing agents such as gold and zinc, and eluents / etchants for processed precious metal products. In recent years, stripping solutions containing cyan ions have been used exclusively for the recovery of gold plating deposits in these processes.

  However, cyanide has a high environmental load and waste liquid treatment costs are increasing year by year. In these fields, development of alternatives to replace cyanion is generally required. Conventionally, it has been attempted to use an etching solution containing iodine ions for etching gold or gold alloy in the field of manufacturing semiconductor wafers for electric circuits. For example, Japanese Patent Laid-Open No. 48-25639 improves the affinity for Au. In order to improve the etching accuracy and to provide the etching with a desired anisotropy, a compound having a specific structure such as a fatty acid alkanol is added to JP-A-2003-109949. It has been proposed to do. However, as described above, these are all related to etching techniques for semiconductor wafers and the like, and it is conventionally known to use a stripping solution containing iodine ions for stripping and recovering gold from the industrial electrical circuit waste. Not.

JP 48-25639 A JP 2003-109949 A

  For example, the present inventors have solved a problem of cyanide ions conventionally used in a system for recovering gold by processing a gold-plated electronic substrate waste material, and as a result of examining a practical release agent that replaces these problems. Focusing on the possibility of using an aqueous solution containing iodine ions as a stripping solution, an effective gold recovery system has been put to practical use.

  In addition, when constructing a practical system for recovering gold using a stripping solution containing iodine ions, the price of iodine is higher than that of sodium cyanide, and the molecular weight of iodine is almost 2 times that of sodium cyanide. The price as a release agent is about 5 times that of sodium cyanide. In order to reduce the burden on the environment in the gold recovery system, the present inventors employ a stripping solution containing iodine ions in the gold stripping process, and further strip the gold to solve the economic problem. A system including a step of circulating iodine used in the waste liquid after use to the gold stripping step and reusing it as a gold stripper was constructed.

According to the present invention, when a waste material containing a gold deposit such as an electric circuit board is processed to separate and recover the gold, the gold plating waste material is treated with an aqueous solution for removing the gold containing iodine and iodine ions. The iodine solution containing the separated gold is treated with zinc to reduce and precipitate the gold. The precipitated gold is filtered and washed, dissolved and extracted as chloroauric acid with aqua regia, and then sodium hydroxide. Gold oxide is purified and recovered by reducing the gold oxide by back-extraction by (Claim 1).
Further, in the gold recovery method, an aqueous solution containing iodine ions after reduction and precipitation of the gold is treated with an anion exchange resin to adsorb and separate iodine ions, and this is used as a form of the aqueous solution for removing gold. The iodine ions are circulated and used by circulating them (Claim 2).

  According to the present invention, in order to separate and recover gold from gold plating waste material, etc., the gold plating waste material is treated with an aqueous solution for removing gold containing iodine and iodine ions to separate the gold as gold iodide, and the peeled gold The iodine solution containing is treated with zinc to reduce and deposit gold, and then this is converted to chloroauric acid by extraction and separation with aqua regia and gold oxide by back extraction with sodium hydroxide, and then reduced to purify and recover gold. As a result, the load problem on the environment derived from the use of cyanide ions in the conventional gold recovery operation is greatly reduced.

  Furthermore, the treatment waste liquid after reducing and precipitating gold with zinc contains a considerable amount of iodine ions, this is adsorbed and separated by an anion exchange resin, and circulated in the form of the gold stripping liquid. Since it is reused for peeling gold, the expensive material cost of iodine can be reduced and the iodine content in the treatment waste liquid can be reduced to reduce the burden of waste liquid treatment.

Best Mode for Carrying Out the Invention

  Embodiments of the method of the present invention will be described below for each step.

1. Stripping aqueous solution containing iodine and iodine ions and gold stripping thereby (i) Dissolution of iodine Although iodine I ₂ does not dissolve in water, it dissolves in water when potassium iodide KI is present as shown in the following formula.
KI + I = KI ₃ (1)
^{_{KI 3 = K + I 3 -}} (2)
I ₃ = I + I ₂ (3)
Reaction formulas (1), (2), and (3) are in an equilibrium relationship, and when iodine ions are consumed, the equilibrium proceeds to the right.

(Ii) Gold peeling The solubility of iodine in water is as low as 0.336 g / (L) at 25 degrees, but dissolves in an aqueous solution containing potassium iodide.

  To measure the amount of gold dissolved by iodine ions, it is easy to add up to 6 times by adding 0.4 g of gold foil at a time while stirring 200 ml of an aqueous solution containing 0.5 g of iodine in an aqueous solution containing 20 g of potassium iodide. In the seventh time, it takes time to dissolve, and in the eighth time or more, the gold foil remains undissolved. As a result, as shown in Table 1, the substantial amount of dissolved gold was 0.27 g / 200 ml.

The equivalent amount of dissolved gold is 1.40g / 196.97 = 0.00711mol
The equivalent of iodine is 2.50g / 126.91 = 0.00985mol
The equivalent ratio of gold and iodine is 0.00985mol / 0.00711mol = 1.39. From the above reaction formula (3), the dissolution of gold is also dependent on potassium iodide, and assuming that half of the equivalent of iodine is involved in the dissolution of gold, the equivalent number of total iodine is the total equivalent of iodine 0.00985 mol + 0.00985mol
× 1/2 = 0.01473mol
Is calculated. The equivalent ratio of gold to total iodine is
I / Au = 0.01473mol / 0.00711mol = 2.07 / 1.00
Thus, the composition of dissolved gold is estimated to be AuI ₂ ⁻ .

  Although the gold foil is not dissolved in the potassium iodide aqueous solution, as shown in Table 1, when the amount of iodine added is increased, the amount of gold foil dissolved is increased as the amount of iodine ions is increased.

(Table 1)
Table 1: Amount of iodine added and amount of gold foil dissolved (iodine added to 200 ml of a solution containing 20 g of KI)

  A gold plating layer having a gold thickness of 1 μm or less is easily dissolved in the same manner as a gold foil. However, as the plating layer becomes thicker, the time required for dissolution becomes longer. For example, 5 g of gold bullion is immersed in 500 ml of a solution of 13 g of iodine in 40 g of potassium iodide, and if left for 3 days, about 3.82 g is dissolved. That is, the larger the surface area of gold, the shorter the time required for dissolution of gold.

The redox potential of gold by iodine ions is as follows (Chemical Handbook II, p473, revised 3rd edition, Maruzen, 1984).
Au + 2I ^-- 2e = AuI ₂ ^- Standard potential Eo = 0.578V

  This reaction is similar to that of gold cyanide, indicating that the presence of an oxidant promotes the dissolution reaction. When the solution is added with stirring while dissolving the gold foil, the dissolution easily proceeds because oxygen in the air acts as an oxidizing agent.

The oxidizing action of oxygen in the air depends on the pH of the aqueous solution and is expressed as follows.
E = 1.23-0.0591 pH (however, when calculated at a temperature of 25 degrees, E is a redox potential)
The potentials calculated by changing the pH value are shown in Table 2 below.

(Table 2)
Table 2: pH and potential of aqueous solution (V)

  If the potential difference is 0.2 to 0.4 V in the oxidation reaction, the reaction is considered to proceed quantitatively, and it is presumed that the peeling of gold proceeds more effectively on the acidic side than pH 7.

In addition to oxygen in the air, an iodate IO ₃ ^- or a combination of iodate and hydrogen peroxide may be considered as an oxidizing agent. Iodic acid is a strong oxidant, but at pH> 5 the reaction as an oxidant is slow.
^{_{IO 3 - + 6H + + 5e}} → 1 / 2I 2 + 3H 2 O Eo = 1.20V

In addition, neutral iodic acid generates oxygen intermittently in the presence of hydrogen peroxide and oxidizes iodine to iodic acid. The reaction is
2HIO ₃ + H ₂ O ₂ → I ₂ + 6H ₂ O + O ₂
I ₂ + H ₂ O ₂ → 2HIO ₃ + 4H ₂ O
Indicated by

  When iodic acid is used as an oxidizing agent, the iodine concentration in the stripping solution is increased, and the reaction as an oxidizing agent is promoted to increase the amount of dissolved gold. In any case, the oxidizing agent is preferably used in an acidic state.

  After the gold is peeled off, the electronic board is washed in three stages of countercurrent, drained, and handed over to the copper collector as a valuable resource.

2. Reduction of gold iodide To reduce gold from an iodine solution from which gold has been removed using metal zinc powder, acidic and alkaline reduction reactions are assumed. These redox potentials are shown below.

The oxidation-reduction potential of metallic zinc powder is acidic Zn-
2e = Zn ²⁺ Eo = −0.763V
Alkaline Zn + 2OH ^-- 2e = ZnO ₂ ^2- + 2H ⁺ Eo = -1.22V
It is. The redox potential of gold iodide is
^{_{AuI 2 - + e = Au +}} 2I - Eo = 0.578V
Therefore, the difference in oxidation-reduction potential between zinc and gold iodide has a great influence.
AuI Under acidic ^{_{2 - + e = Au + 2I}} - Eo = 0.578V
1 / 2Zn − e = 1 / 2Zn ²⁺ Eo = −0.763V
^{_{AuI 2 - + 1 / 2Zn =}} Au + 1 / 2ZnI 4 2-
Potential difference 0.578 − (−0.763 × 1/2) = 0.960V

AuI ₂ in an alkaline ^{- + e = Au + 2I -} Eo = 0.578V
1 / 2Zn + OH ^-- e = 1 / 2ZnO ₂ ^2- + H ⁺ Eo = -1.22V
^{_{AuI 2 - + 1 / 2Zn +}} OH - = Au + 2I - + 1 / 2ZnO 2 2- + H +
Potential difference 0.578 − (− 1.22 × 1/2) = 1.188V

  Since the potential difference is more alkaline than acidic, it is preferable to perform zinc reduction with alkalinity, but considering the recovery of iodine with an anion resin as described later, it is preferable to perform zinc reduction with acid, It is selected according to the purpose.

In the case of gold cyanide, the redox potential is
^{_{Au (CN) 2 - + e}} = Au + 2CN - Eo = -0.611
Thus, it is alkaline and reduced by zinc powder.

  The redox potential of gold cyanide is lower than that of gold iodide. This is because the coordination bond of the gold cyanide complex is larger than the coordination bond of the iodinated gold complex, and tends to be difficult to reduce. Further, the reduction of gold cyanide with an acid makes operation difficult due to the vaporization of hydrogen cyanide.

3. Gold refining Gold that has been reduced with zinc powder in the gold stripping process is filtered and washed, dissolved in aqua regia, and extracted with methyl isobutyl ketone (MIBK) to extract HAuCl ₄ chloroaurate. By this operation, the denitration operation performed by treating nitric acid in the aqueous solution with concentrated hydrochloric acid can be omitted.

When the extracted chloroauric acid is back extracted with a sodium hydroxide solution, a black precipitate is formed. The generated black precipitate is presumed to be gold oxide Au ₂ O ₃ .
HAuCl ₄ + 4NaOH = Au (OH) ₃ + 4NaCl + H ₂ O
2Au (OH) ₃ = Au ₂ O ₃ + 3H ₂ O
The black precipitate is dissolved in dilute hydrochloric acid, and sodium sulfite is added while heating to reduce and condense gold. After washing with warm water, it is dried at 105 degrees and then melted in a graphite crucible to form an ingot.

4). Recovery of iodine (and zinc) from aqueous solution after gold reduction The filtrate obtained by reducing gold iodide to zinc to remove gold contains ions such as potassium and sodium in addition to iodine and zinc. In order to recover iodine and zinc, especially iodine, from these aqueous solutions, the filtrate is once collected in the storage tank 1 (system flow diagram, middle column).

  The iodine ion is an anion exchange resin, and the zinc ion is concentrated using a cation exchange resin as described later.

  The filtrate after the gold reduction is adjusted to pH = 4 with sodium hydroxide when the gold is reduced acidic and with dilute sulfuric acid when the gold is reduced and precipitated alkaline. The pH-adjusted aqueous solution adsorbs iodine ions in a fluidized bed system in which the solution is injected from the bottom of a resin tower packed with an anion exchange resin having a quaternary ammonium functional group. The ion exchange resin that adsorbs iodine ions accumulates in the lower part of the resin tower. The adsorbed iodine ions are eluted with 0.05 molar sodium chloride solution, and the iodine ions are oxidized to iodine at 500 to 600 mV, filtered and washed with water to recover crude iodine. As the gold stripping solution, crude iodine can be recycled and reused as a stripping solution in the gold stripping step without being purified.

When an iodic acid solution or hydrogen peroxide solution is used as the oxidizing agent, it is reduced with sodium hydrogen sulfite, and the resulting iodine is dissolved as iodine ions by the coexisting potassium iodide.
^{_{2NaIO 3 - + 5NaHSO 3 → 3NaHSO}} 4 + Na2SO 4 + H 2 O + I 2

The solution after the adsorption and separation of iodine ions with an anion exchange resin is collected in the storage tank 2 (the right column of the flow diagram, the solution is injected from the bottom of the resin tower packed with a cation exchange resin having a carboxyl group as a functional group to make zinc Zinc adsorbed on the resin is eluted with 0.1 molar dilute sulfuric acid solution, and excess acid is neutralized with zinc oxide or zinc powder.The neutralized solution is introduced into the electrolytic cell and contains silver at the anode. A lead plate is used, and an aluminum or zinc plate is used as the cathode, and zinc is electrolytically deposited at a current density of 1.70 to 4.00 A / dm ^{2. If an} aluminum plate is used as the cathode, the deposited zinc must be peeled off. The solution after zinc electrolysis can be circulated and used as an eluent for a cation resin adsorbing zinc ions.

As a method for separating iodine ions and zinc ions other than the resin adsorption method, it is also possible to add copper sulfate and separate it as sparingly soluble copper iodide CuI (solubility product logKsp = -12.03). The separated copper iodide is acidic with sulfuric acid and is separated as iodine using the oxidizing agent manganese dioxide MnO ₂ or trivalent iron oxide Fe ₂ O ₃ . In this case, it is necessary to discard manganese sulfate and iron sulfate after recovering iodine.

  Gold-plated electronic board waste contains approximately 1/100 to 1/1000 gold by weight. When this waste material is immersed in a release agent containing potassium iodide and iodine heated to 30 to 40 degrees and vibrated to come into contact with oxygen in the air while the peeling solution is attached to the gold surface, the gold is peeled off. Is done. Waste material after the peeling of gold is transferred to a washing tank and washed in three counter-currents. A new waste material is immersed in the peeling tank and the peeling of the gold is continued in the same manner. As the concentration of dissolved gold in the stripping solution increases, the dissolution rate decreases. In this case, when the sodium iodate solution or sodium iodate and 3% hydrogen peroxide solution are added as oxidizing agents, the peeling of gold proceeds.

  If peeling does not proceed easily even when an oxidizing agent is added, the stripping solution is replaced. The amount of gold dissolved in an equilibrium state in an aqueous solution of 2 (L) containing 25 g of iodine and 20 g of potassium iodide is 55 g, almost 93.1% of the theoretical amount of 59.1 g.

  80 g of sodium hydroxide is added to the peeled solution, and 15 g of zinc powder is added thereto and stirred. The black zinc powder gradually turns white and the solution becomes suspended. The precipitate is matured, so it is left overnight, filtered and washed with water. Transfer the precipitate to another container and dissolve it in aqua regia. Thereafter, the process proceeds to a gold purification step in which solvent extraction is introduced, and gold is purified.

  The aqueous solution obtained by washing the zinc-reduced precipitate with filtered water is transferred to the storage tank 1 and adjusted to pH 4 with dilute sulfuric acid. The prepared solution is injected at a flow rate of 100 ml / 30 minutes from the bottom of the resin tower packed with 70 g of anion exchange resin to adsorb iodine ions. The adsorbed iodine ions are eluted using 0.1 molar sodium chloride solution as an eluent. When the eluted iodine solution is oxidized with a dilute solution of hydrogen peroxide so as to become 500 to 600 mV, iodine is precipitated. The precipitated iodine is washed with water and reused as a gold stripping solution. The amount of iodine deposited is about 98%.

The leaked liquid adsorbed with iodine ions is transferred to the storage tank 2 and injected at a flow rate of 100 ml / 30 minutes from the bottom of the resin tower packed with 70 g of cation exchange resin to adsorb zinc ions. The adsorbed zinc ions are eluted with 0.1 molar dilute sulfuric acid. Eluent is introduced into the electrolytic bath having the lead plate to the anode, a zinc plate as a cathode, electrolysis at a current density of 2A / dm ^2. The electrolytic solution circulates between the electrolytic cell and the storage cell, and adjusts the zinc concentration in the electrolytic cell.

  Gold peeling with an iodine ion solution was performed in the same manner as in Example 1, and metal zinc powder was directly added to the peeled solution to reduce the gold. The amount of gold deposited was 56 g, almost 94.7% of the theoretical amount of 59.1 g. The operation for recovering iodine and zinc was performed in the same manner as in Example 1.

  When recovering gold from gold-plated electronic circuit board waste, etc., it is possible to reduce the burden on the environment by using iodine ions instead of cyan ions in the conventionally used stripping solution. By separating the iodine used in the treatment from the treatment waste liquid and circulating it, the material cost of expensive iodine is reduced.

It is a flowchart which shows the process of the system by the gold | metal stripper which circulates and uses the iodine ion of this invention.

Claims (2)

A gold stripping process in which gold plating waste material is treated with an aqueous solution for stripping gold containing iodine and iodine ions to separate gold as gold iodide;
A gold precipitation step of depositing gold by reducing the iodine solution containing the separated gold with zinc;
Iodine having a gold refining step of filtering and washing the deposited gold, dissolving and extracting as chloroauric acid with aqua regia, then converting to gold oxide by back extraction with sodium hydroxide, and reducing and purifying the gold A method for recovering gold using a gold remover that circulates ions. An iodine ion separation step of adsorbing and separating iodine ions by treating an aqueous solution containing iodine ions after the reduction and precipitation of gold in the gold precipitation step with an anion exchange resin;
The method for recovering gold with a gold stripping solution that circulates and uses iodine ions according to claim 1, further comprising an iodine circulation step of circulating the iodine ions in the form of the aqueous solution for stripping gold into the gold stripping step.

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